1. Field of the Invention
The present invention relates to a method for the rotary sawing of brittle and hard materials, in particular of those having diameters exceeding 200 mm, into thin wafers by means of an annular saw and apparatus for performing the method.
In particular, the present invention relates to a method for the sawing of brittle and hard materials, in particular of semiconductor crystalline ingots or blocks, by means of an annular saw into thin wafers, in which the workpiece is moved towards the cutting edge of the saw while rotating.
2. The Prior Art
Crystalline ingots or blocks of semiconductor materials such as, in particular, silicon, germanium or gallium arsenide, are predominantly cut by means of annular saws into the thin wafers which are needed to produce devices. The saw blade of annular saws has the form of a circular disk and is clamped into a frame at its outer circumference. The inner circumference of the disk forms the cutting edge. As a rule, it is composed of a coating which is drop-shaped in cross-section and is composed of a metal such as, for example, nickel in which hard-material granules composed of diamond or boron nitride are bonded by electroplating. The grinding action of the rotating cutting edge on the workpiece produces the removal of semiconductor material. Depending on the design of the machine, the saw blade rotates in a horizontal or in a vertical plane. Taking account of the kerf width to be expected, the crystalline ingot is fed into the central opening of the circular disk to such an extent that wafers having the desired thicknesses are cut off when the ingot is then moved towards the cutting edge in an advancing movement parallel to the plane of rotation. Normally, the crystalline ingot is rigidly mounted on a feed carriage during this operation.
The requirements imposed on the quality of the wafers, which as a rule have a thickness of 0.1 to 1 mm, are very high. The maintenance of permitted tolerances relating to deviations in the plane-parallel geometry makes it very difficult, in particular, to produce wafers from crystalline ingots having large diameters exceeding 200 mm. The increase in saw blade outer diameter necessary for sawing such workpieces requires a thickness reinforcement of the saw blade since, without the thickness reinforcement, the latter would depart in an impermissible way from the ideal cutting line even in the event of slight differences in the forces acting perpendicularly to the plane of the saw blade as a consequence of deficient rigidity. However, an increase in the clamping forces to prevent an elastic deformation of the saw blade is possible only in the case of thickness-reinforced saw blades which are able to withstand these forces. Associated with the use of thicker saw blades, however, is an extremely disadvantageous material loss because of the increased kerf. If, on the other hand, cutting is carried out with a saw blade which is only enlarged radially, the wafer geometry defects, which usually manifest themselves in a thickness variation and/or in curved wafer planes familiar to the person skilled in the art under the descriptions "warp" and "bow," are unacceptably increased.
Although it is possible to correct such geometry defects, for example, by GS (grinding/slicing) cutting, the material loss due to correction increases with the severity of the defect. GS cutting is described in U.S. Pat. No. 4,896,459. The method combines the sawing operation with an end-face grinding of the crystalline ingot. As a result, every wafer cut off acquires a flat reference face and can be ground parallel to this plane in a subsequent treatment step.
U.S. Pat. No. 3,025,738 and U.S. Pat. No. 3,039,235 disclose the fact that the saw blade outside diameter needs only to be half as large if the workpiece is presented for sawing while rotating around the longitudinal axis instead of being mounted rigidly on a feed carriage. The machining of ingots having crystal diameters exceeding 200 mm is accordingly still possible with conventional saw blades whose dimensions would already be too small if the workpiece were fed without self-rotation, but which are still sufficiently rigid to keep geometry defects within the tolerance range.
The method described as rotary sawing has, however, serious disadvantages which arise from the fact that, because of the brittleness of the material, and as a consequence of the prevailing torsional and inertial forces which make themselves felt, in particular, in wafers having diameters of over 200 mm, the residual joint between ingot and wafer becomes so labile towards the end of the sawing operation that a spontaneous breakage often occurs. As a rule, this leaves behind in the center of the wafer or in the end face of the ingot an undesirable depression (center damage) whose removal requires a substantially more expensive subsequent grinding than in the case of the normal after-treatment of the wafer surfaces. It is not uncommon for the center damage to extend so deeply into the interior of the wafer that the entire wafer is unusable. The uncontrolled breaking-off of the wafer is always accompanied by an unacceptable material loss.
All the efforts hitherto undertaken by those skilled in the art to counteract center damage had the objective of completing the cutting operation using the annular saw without random breakage of the residual joint between crystalline ingot and wafer. Thus, for example, German Offenlegungsschrift 30 10 867 presents a take-up device which rotates synchronously with the crystalline ingot and which is intended to stabilize the wafer adequately until it is separated from the ingot in a controlled manner. Such a safety measure is, however, complicated and costly and in particular, is unreliable in the case of wafer diameters exceeding 200 mm.
As a consequence, therefore, the sawing method which has been mentioned and in which the crystalline ingot is fed without self-rotation is routinely used in industry. Normally, the workpiece is cemented onto a sawing strip made of graphite or carbon so that even after being cut from the ingot, the wafer is held in position. If the cutting edge starts to penetrate the sawing strip, a vacuum pick-up is brought up, applies suction to the wafer and transports it away from the sawing area after the strip has been cut through. Although this procedure makes center damage to the wafer or the ingot impossible, it does not prevent the problems with the wafer geometry which have been mentioned and which arise in sawing workpieces having diameters exceeding 200 mm.